During information transmission via electromagnetic signals there will occur large local variations in signal strength, due to interference between signals received directly, and signals which have been reflected against various objects, e.g. buildings, or signals which have been reflected against the terrain.
In order to reduce these problems, so called diversity reception can be used, which means reception using at least two antenna units, the received signal strength of which is monitored so that maximal signal strength can be used for reception. The so far most widely used form of diversity reception is so called space diversity, which means using at least two antennas separated in space. When using directive antennas with sector spacing, several antennas are needed, thus taking up a great deal of space. This means high production and installation costs, and a negative esthetic influence, for example base stations for mobile telephony systems, which in many cases are located in densely populated areas. In order to obviate these problems, polarization diversity can be used instead, where one and the same antenna unit can be used with dual polarization. This enables reception of signals, the polarization of which has been twisted due to reflections of the electromagnetic signal against surrounding objects. The antennas are often constructed using so called microstrip technology. Known dual polarized microstrip have the drawback of a complex feeder network, where each polarization requires one feeder network.
DE 4 239 597 A1 shows a so called microstrip antenna, with dual polarization, in which the antenna elements for the different polarizations are located in different layers. This leads to a complex structure, with a large amount of layers for both the antenna elements and the feeder networks, which causes high costs.
The object of the present invention is a device in antenna units, in which the feeder network is greatly simplified and compressed.